What the heck is bias anyway?

I'm getting a new tube amp that will need to be biased. The process of doing it seems fairly straight forward, but I am curious: what are you actually doing when you bias an amp? In english, for us technical Ludites.
Grimace what tube amp will you be biasing?
Cary v12. Apparently you can use a meter, or just turn the screws until the lights come on. Seems easy enough.
HEHEHE. Seems to be a few sublimal messages going on here.

PBNaudio. Stanwal's message was rather cute however Atmasphere's was even better. Hey,I hope you guys know how rebias those output transistors when required.
Grimace nice amp;what output tubes will you be running?
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